Induction heating



1963 R. J. SCHROYER 3,363,079

INDUCTION HEATING Filed Feb. 15, 1965 2 Sheets-Sheei 1 INVENTOR ROBERT J. SCHROYER ATTORNEYS Jan. 9, 1968 J, SCHRQYER 3,363,079

' INDUCTION HEATING Filed Feb; 15, 1965 2 SheetsSheet 2 INVENTOR ROBERT J. SCHROYER W www? ATTORNEYS United States Patent 3,363,079 INDUCTION HEATING Robert J. Schroyer, Fairiield, Ohio, assignor to The Black Clawson Company, Hamilton, Ohio, a corporation of Ohio Filed Feb. 15, 1965, Ser. No. 432,537 4 Claims. (Cl. 219-1t).49)

ABSTRACT 0F THE DISCLUSURE This invention relates to induction heating and more particularly to induction heaters for plastic material converters and extruders.

Although induction heating has been used with plastic material extruders for maintaining a desired extruding temperature, difliculty has been encountered when such heaters are used in relatively high temperature applications due to the fact that the electric coil is often subjected to temperatures which cause destruction of the coil insulation. It has accordingly been necessary to isolate the electric flux-inducing coil from the die body being heated in order to permit the coil to work within safe temperature limits. Although arrangements have been made for isolating such coils which have proved highly successful, these arrangements have often resulted in an induction heating assembly which was both complicated and expensive to make and which, nevertheless, had upper temperature limits beyond which coil life is shortened.

The induction heating units of this invention eliminate many of these problems by the employment of an induction heating coil which may be operated at the same temperature as the body being heated without deterioration or destruction of the coil or the coil insulation. The apparatus of this invention permits the coil to be placed considerably closer to the body being heated, and even potted or embedded within the body. The preferred form of the coil of this invention consists of a plurality of turns of an anodized aluminum conductor, preferably aluminum tape or foil, which have been wound in contiguous relation to each other about the device or die body to be heated, with adjacent layers or turns being electrically isolated from each other by the anodized layers on the surface of the turns of aluminum. Preferably, the coil is supported on a coil form of non-magnetic material, although it is within the scope of the invention to use such a coil without any coil form support.

In one embodiment of this invention a flux guide is formed by wrapping a plurality of turns of sheet iron material directly about the coil, and preferably directly supported on the coil. This construction has the advantage of retaining the flux guide in close proximity to the coil and to the body which is being heated by the coil.

It is therefore an important object of this invention to provide an induction heater as outlined above which is suitable for use at elevated temperatures without deterioration of the induction heating coil or its insulation.

Another important object of this invention is the provision of an induction heater for a plastic material extruder in which the heating coil is formed with a plurality of turns of anodized aluminum material.

A further object of this invention is the provision of an induction heater having an induction heating coil formed as a continuous strip of flat anodized aluminum tape or foil wound in convolutions with adjacent turns being electrically isolated from each other by an anodized or oxidized film or layer of aluminum.

A still further object of this invention is to provide an induction heater for a plastic material converter in which the electric current carrying coil includes a plurality of turns of anodized aluminum foil, and which coil may be mounted in direct proximity in the body to be heated or which may he recessed or potted into the body.

Another object of this invention is to provide an induction heater in which a flux guide is formed as a strip of flux conducting sheet material which is wound about the heating coil and which is preferably directly supported thereon in close proximity to the coil and to the body heated by the coil.

These and other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

In the drawings:

FIG. 1 is a fragmentary elevational view, partially in section, of the barrel of a screw-typeextruder and an induction heating unit mounted thereon constructed according to this invention;

FIG. 2 is an enlarged fragmentary section through the barrel and the induction heating unit of FIG. 1;

FIG. 3 is a transverse section through the barrel and the induction heating unit of FIG. 1 with the extruder screw removed, taken generally along the line 33 of FIG. 1;

FIG. 4 is a fragmentary perspective view of an electrical connection to a tape;

FIG. 5 is a perspective view of a tape wound induction heating coil showing the electrical leads; and

FIG. 6 is a fragmentary section showing a further embodiment of this invention showing an embedded coil as applied to the heating of a reducer section for a plastic material extruder.

Referring to the figures of the drawings which illustrate preferred embodiments of the invention, a fragment of a screw-type extruder barrel is illustrated at 10 in FIG. 1. The extruder may be of the type shown in the patent to Kullgren et al. No. 2,944,286. The barrel 10 which is formed of magnetic material such as steel, may be considered as the body member of a plastic material extruder or a plastic converter which is to be heated and maintained at an elevated temperature.

Mounted on the barrel 10 is an induction heating unit indicated generally at 12 which is constructed according to this invention. It is understood that a plurality of the units 12 may be employed along the length of the barrel 1t) and each individually controlled to provide the desired temperature and/or temperature gradient throughout the length of the barrel 10.

The induction heating unit 12 preferably includes means for cooling the barrel to remove excess heat therefrom in the form of a quench chamber 15. The chamber 15 comprises sheet-type stainless steel formed in encircling relation to the barrel generally and coterrninous with the axial length of the unit 12, and includes means forming a water inlet conduit 16 and an outlet conduit 17 which open into the interior or cavity formed by the chamber 15. The axial ends of the chamber 15 have inwardly turned lips 18, as shown in FIG. 2, which are suitably welded to an outer surface of the barrel 10. The quench chamber 15 may be constructed according to the teachings of the patent to Kullgren No. 3,129,459, and designated in such patent by the reference numeral 60.

In the embodiment shown in FIGS. l-3, a spool-shaped coil form 20 is received in encircling relation to at least a portion of the body to be heated and, as shown, encircles the barrel 10 at the chamber 15. The form 20, in fact, is mounted directly on an outer surface of the quench chamher and may be formed with a central generally cylindrical portion 21 and opposite outwardly turned ends 22 for receiving an electric coil therebetween. The coil form may be made of any suitable electrical insulating, high temperature-resistant material such as ceramic materials, or cemented asbestos materials such as sold by Johns- Manville, New York 16, N.Y., under the trade name of Transite. Although the ends 22 of the coil form are preferred they are not necessary to the proper operation of the induction heaters, and the coil form maybe in the form of a section of cylindrical tubing or conduit.

The electric induction heating coil of the embodiments of FIGS. 1 and 2 consists of a continuous strip of fiat aluminum tape 26 (FIG. 4) which has been oxidized, such as by anodizing, along both sides and along the edges. This tape is wound in convolutions about the coil form 20 With an inner turn supported directly on or next to the form and with successive outer turns being supported directly on each other, until the desired number of turns have been made. The turns of the coil which are supported directly on each other are electrically isolated from each other by the anodized films or layers of aluminum on the foil material comprising the coils.

The anodized aluminum tape preferably has a sealed type of anodized film covering both sides and the edges in which the pores are closed. The oxides or anodized protective film may be produced by any suitable electrolytic or chemical process. The breakdown voltage of the anodized layer depends upon the thickness of the layer, and it has been found that relatively thin layers of anodized film provides sufiicient protection to prevent layer-to-layer electrical shorting. Although the breakdown voltage of oxide or anodized layers on aluminum has been found to be substantially linear with film thickness, and may vary anywhere from less than 100 volts to more than 300 volts, the manner in which the turns are wound on the coil 25 presents two such layers of anodized aluminum film between adjacent conductive turns due to the fact that the aluminum tape is anodized on both sides. Thus, the breakdown voltage is effectively doubled.

Since the coil is wound in convolutions, successive or adjacent turns are of very nearly the same potential minimizing any tendency to breakdown. For instance, when 200 turns of aluminum foil are used with a 440 volt alternating current supply, there is only a nominal 2.2 volts per turn voltage distribution throughout the coil. Accordingly, although cracks in the anodized coating may occur due to the bending of the tape, particularly where the coil 25 is wound about a small radius, such cracks have little or no deleterious effect upon the operation of the coil due to the fact that there remains an air space between adjacent turns, and further due to the fact that since the foil is preferably anodized on 'both sides, the exact coincidence of cracks on adjacent turns is minimized.

The employment of anodized aluminum foil or tape has been found to be of particular advantage for use in induction heaters since the melting point of aluminum is about 660 C., which is well above the practical limits for plastic material. The aluminum oxide anodized layer has a much higher melting point of approximately 2000 C. It is also known that the dielectric constant of an anodized layer, which lies between 7 and 12, does not deteriorate at high temperatures but even tends to increase with temperature. While the breakdown voltage may decrease some with temperature, this has not been found to be a disadvantage.

Preferably, the foil orv tape 26 has a width corresponding to the width of the coil form. In the embodiment shown in FIG. 1, the unit 12 comprises a pair of forms 20 on each of which is wound a coil 25. The width of the foil material of the coil 25 is thus preferably coincident with the width of the coil form although it is within the scope of this invention to use narrower tapes which are wound both in helix and in convolutions about the coil form to obtain the desired number of turns.

The thickness of foil depends only on the cross sectional area required for the conductor according to good electrical engineering practice. For example, aluminum foil two mils thick and less may be used. It is also within the scope of this invention to use thick or heavy foils such as sheets in excess of seven mils in thickness. However, where such a single tape may be found to be too thick or stiff to be wound easily, the turns may comprise two or more thinner foil tapes wound together in parallel to form a single effective conductor. This is analogous to the procedure of winding in hand two or more strands of a standard copper conductor.

A further advantage of the arrangement of this invention is that since each turn of the coil is wound on top of the preceding turn, the low and high voltage windings are electrically concentric and are balanced. Accordingly, there is no induced axial force affecting the coil such as is common With wire wound coils. Also, due to the thinness of the insulation, which may be in the order of microns, a high space factor is achieved. That is, the available coil space is utilized to a high degree by conductor material as compared to the total space. Also, it has been found that the tape wound coil has an improved power factor when compared to the conventional induction heating coil.

This invention further provides a flux guide 30 formed in surrounding relation to the induction coil. The guide 30 consists of a plurality of turns of suitable magnetic iron sheet material such as transformer steel. The turns of the flux guide 30 are wound in convolutions, similar to those of the coil 25, with an inner turn supported directly on or adjacent the outer surface of the coil 25 and with successive turns wound directly on each other. It may, in some instances, be preferable to insert or wrap a thin layer of insulating material between the flux guide 30 and the outer surface of the coil 25 to prevent the possibility of electrical breakdown due to the fact that the full line potential may occur across these elements. Suitable materials for this purpose include fiberglass tape impregnated with silicone and sheet mica. The wrapped turns forming the flux guide may be split as by cutting through the guide longitudinally of the barrel as indicated at 31 in FIG. 3, to reduce eddy current losses. One such guide 30 is preferably used for each coil 25 supported on the coil and in surrounding relation to the coil.

With the arrangement shown, the flux guide 30 is wound and formed in close proximity to the coil 25 which, in turn, is maintained in close proximity to the barrel 10 to be heated. The overall construction is one of high efficiency as well as low initial cost of production. The induction heating coil 25 will operate very nearly at the same temperature as the barrel 10, With an upper limit which is determined only by the melting point of the aluminum or by other components of the converter.

FIGS. 4 and 5 show typical electrical connections to the ends of the tape 26 and provide the electrical connector means by which line power may be applied across the coil. Each electrical connection may consist of a striplike connector 36 which is received over one of the ends of the tape 26, mechanically crimped, and pressurewelded to the tape to form both a suitable mechanical bond as well as an electrical connection through the anodized coating. A lead wire 37 may conveniently extend from an end of the strip 36. In FIG. 5, the strips 36 at each end of the tape are shown as being bent at right angles to the axis of the coil with the terminal leads 37 extending radially outwardly therefrom. Ceramic insulator beads 38 may be used over the lead wires 37 for effective electrical isolation.

The electric heating units of this invention may also be used with the coil recessed into a suitable cavity formed for this purpose within the extruder body to be heated. The body of the extruder, itself, may form the flux guide return path for the coil. An example of this arrangement is shown in FIG. 6 which comprises a reducer body 54} supporting the usual breaker plate 51 mounted to receive the plastic melt 0n the end of the barrel or on any other similar extruder barrel. The body 50 is shown as being formed with an annular external recess 55 within which an annular tape wound coil 56 is received. The coil 56 is formed of a continuous strip of aluminum tape or foil made as described above in connection with the coil 25.

The coil 56 is shown as being mounted directly in the cavity 55 with-out the use of an auxiliary coil form such as the form 20 of FIGS. 1-3. However, the coil 56 is preferabiy wrapped with a suitable electric insulating material such as the silicone impregnated glass tape 58.

Means for forming a return flux path may comprise, as shown in FIG. 4, an integral part of the extruder body and in this illustration is an annular ring 60 which is L-shaped in section and mounted in surrounding relation to the coil 56 and secured to the body 50 'by screws 61. When assembled, the coil 56 is submerged or embedded within the body to be heated and surrounds at least a portion of such body. The heaters shown in FIG. 6 may be used to create in the body 50 temperatures in the order of those which can be created in the body 10 of FIGS..1-3 without deterioration of the coil or of the electrical insulation formed therebetween by the anodized layers of the individual turns of aluminum foil.

Accordingly, this invention provides induction heaters which are useful at temperatures in excess of the maximum temperature at which ordinary induction heating coils may be used. The coils themselves may be placed in the closest possible proximity to the body heated for eflicient flux coupling. The induction heaters do not require heat isolation, have excellent resistance to shocks and physical abuse, and are relatively inexpensive to make.

While the forms of apparatus herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. In an induction heater having plastic material extruder barrel having a non-magnetic water cooling jacket mounted on said barrel in surrounding relation thereto, the improvement comprising an induction heating coil comprising a plurality of turns of anodized aluminum foil wrapped in convolutions about each other, means mounting said coilv in closely surrounding relation to said jacket, means for applying alternating current power to said coil, and a plurality of turns of transformer-grade sheet steel wrapped in convolutions about said coil and having a width corresponding to the width of said coil forming a flux guide for said induction heating coil.

2. In an induction heater having plastic material extruder barrel having a non-magnetic water cooling jacket mounted on said barrel in surrounding relation thereto, the improvement comprising means forming an electrically isolating coil form received in surrounding relation on said jacket, an induction heating coil comprising a plurality of turns of anodized aluminum foil wrapped in convolutions about said form, electrical connector means for applying alternating current power to said coil, and a plurality of turns of transformer-grade sheet steel wrapped in convolutions about said coil forming a flux guide for said induction heating coil.

3. An induction heater for a plastic material extruder having a body member subject to heating by magnetic induction comprising a non-magnetic, non-conductive high temperature-resistant annular coil form received in encircling relation to at least a portion of said body member, a continuous strip of anodized aluminum foil wound in convolutions on said form forming an annular induction coil having an inner turn supported on said coil form and each additional turn wound on top of the preceding turn and being electrically isolated from each other by the anodized films thereon, means on said coil comprising electrical connections for applying alternating current thereto, and an annular flux guide formed of sheet iron material having a width corresponding generally to the Width of said foil and being supported in surrounding relation to said coil.

4. An induction heater for a plastic material extruder having a body member subject to heating by magnetic induction comprising a non-magnetic, non-conductive high temperature-resistant coil form received in encircling relation to at least a portion of said body member, a continuous strip of aluminum foil which is anodized on each side and wound in convolutions on said form forming an induction Coil having an inner turn supported on said coil form and each additional turn wound on top of the preceding turn and being electrically isolated from each other by the anodized material thereon, means on said coil comprising electrical connections for applying alternating current thereto, and a flux guide comprising a plurality of turns of fiux-conducted sheet material having a width corresponding generally to the width of said foil and wound in convolutions about said coil and being directly supported thereon.

References Cited UNITED STATES PATENTS 2,604,517 7/1952 Brennan 2l9--10.79 X 2,868,938 1/1959 Barfield et al. 219-1049 2,998,583 8/1961 Worchester 336-223 3,129,459 4/ 1964 Kullgren et al. 219-10.49 X 3,283,190 11/1966 Applegate 31043 X RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner. 

